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When Christians put him there - for what ever reason. My guess is they needed an adversary and said adversary needed his own special day.

{shrug}

Again, what a few hate filled whack jobs have to say in their efforts to fleece the gullible has nothing to do with Christianity or Christians. And, again, it's like describing those Neo-Nazis that go on about worshipping Wotan as representing Neo-Paganism. Would you think it fair to most Neo-Pagans if a Christian were to say, regarding racist hate crimes, "My guess is Neo-Pagans needed an adversary, so they put Blacks and Jews there..."

Being a Celtic holiday when did christains take over it and claim its a day to celebrate their devil?

There are a couple levels of complexity at play here, and your assumptions are not quite accurate.
First things first, Halloween is not interchangeable with Samhain. They came about in different cultures, for different reasons, and only became intertwined in a few specific cultural areas at a fairly late date.
Samhain is a Celtic festival, specifically Gaelic. Though it has comparative traditions in Wales and Cornwall. Samhain is likely pre-Christian in origin and has significance in Gaelic mythology. It probably commemorated the transformation from Autumn into Winter and the last of the major harvests. It was placed, by custom, around what we define as early November; it probably varied a bit based on astronomical or climatological indicators.
Halloween came about in relation to All Saint's Day, as an evening of celebration the night before All Hallow's. The feast of All Saints originally was placed in early May, by the Pope, as a day of pilgrimage to commemorate the interment of certain Saints' relics in Rome. It wasn't changed to November 1st until almost two centuries later, out of public health concerns because Rome was struck with a really terrible fever epidemic in the summer. It had basically nothing to do with Samhain.

Now, yes, elements of Samhain were grafted onto Halloween. But it's not like them big, bad Christians came to Ireland to steal the folks' holidays and sewed a cross on 'em. These were cultural customs that were basically undisturbed in the Christianization process. Yes, certain mythological elements and the gods were altered significantly; the development of fairy folklore is probably, at least in part, an adjustment of polytheistic gods into lesser spirits that could be accepted by Christian folklore. But by and large, Samhain was not drastically altered. It remained a popular celebration up through the Early Modern Period. Instead, what happened is that because Samhain and Allhallow's were celebrated simultaneously, they exchanged concepts and practices. The same applies to Halloween, which eclipsed both as a popular night for revelry before the start of the solemnity. The overly strong focus on death and mortality in Halloween probably owes far more to All Saint's Day than it does to Samhain. Keep in mind, this was pretty much only in the British Isles, specifically the Gaelic countries. The imports from Samhain are likely the practice of divination, apple-bobbing games, the emphasis on Autumn and the Harvest, and fairy lore. The ubiquity of ghost stories are probably a point where Samhain's fairy folklore crosses paths with Allhallow's emphasis on the dead.

As far as the Devil being placed in relation to Halloween? That's almost certainly an invention of the Protestant Reformation, and its attempt to demonize Catholicism and its macabre sensibilities. It was more of a Protestant dig against Catholics, than it was any kind of a Christian insert against the Celts (who had, keep in mind, converted to Christianity long before the Saxons did).